(1) Field of the Invention
The present invention relates to an image forming apparatus that forms images on the recording medium such as a recording sheet, and especially relates to an image forming apparatus for reproducing gradation with improved correctness. Also, the present invention relates to an image forming method for reproducing gradation with improved correctness.
(2) Related Art
Generally, the digital image forming apparatus processes image data that has been input (input image data) for reproducing improved gradation. Then, an image is formed by the image forming unit according to the image data that has been processed (output image data). One of the data processing is the gradation conversion. In the gradation conversion, the characteristics of color development by the image forming unit are taken into account. The gradation values of the input image data are converted according to a predetermined gradation conversion characteristic so as to reproduce the gradation in the original image as accurately as possible.
The gradation conversion is used in the thermal transfer printer, which forms images using the thermal head. The thermal transfer printer forms an image as follows. According to image data, the thermal head is driven. The thermal head applies thermal energy to an ink sheet. Then, ink on the ink sheet is sublimed to be transferred onto a recording sheet. Even in this case, the gradation values of the input image data needs to be converted in advance according to the color reproduction characteristics of the ink sheet. Accordingly, the gradation conversion is used in the thermal transfer printer.
Note that when an image is formed using the thermal head, thermal energy is applied for each of the pixels of the image. In this case, however, it takes a relatively long period of time for the thermal energy reaches a predetermined amount. Accordingly, it takes a longer period of time for printing one image than other printing methods. This is problematic.
In order to solve the problem and to satisfy user needs to confirm printed image soon, a thermal transfer printer with both functions of normal print mode and high-speed print mode has been recently developed. In the normal print mode, an image is printed at normal speed. On the other hand, the image is printed at a higher speed than the normal speed in the high-speed print mode.
Note that the print density is reproduced according to the amount of thermal energy applied to the ink sheet in the thermal transfer printer. In the high-speed print mode, however, the period of time for applying thermal energy from the thermal head to the ink sheet to reproduce each of the pixels is relatively short. As a result, enough amount of thermal energy may not be applied for some pixels. Accordingly, it is inevitable that the reproduced maximum density is low and the range of reproduced density is narrow in the high-speed print mode compared with the normal print mode.
Although the range of reproduced density is limited to the lower density range in the high-speed print mode, images are formed under the same gradation conversion condition as the normal print mode in the conventional thermal transfer printer. Under the circumstances, the gradation cannot be correctly reproduced and eventually the quality is low in the higher density range of reproduced images.
The problem of the gradation deterioration in reproduced images is not limited to the thermal transfer printer. This problem arises for the image forming apparatus in which the reproduced density depends on the amount of applied thermal energy (referred to the xe2x80x9cthermal image forming apparatusxe2x80x9d in this specification) such as an image forming apparatus that records images on the thermal paper using the thermal head.
It is accordingly an object of the present invention to provide a thermal image forming apparatus by which the image forming speed is increased while the gradation of the output image is maintained as correctly as possible.
The above-mentioned object may be achieved by an image forming apparatus that forms an image on a recording medium, the image forming apparatus that includes: a speed setter that sets an image forming speed according to a user direction; a gradation conversion characteristic selector that stores a plurality of gradation conversion characteristics and selects one of the gradation conversion characteristics, the gradation conversion characteristic corresponding to the image forming speed; a data converter that converts gradation values of input image data to generate output image data according to the selected gradation conversion characteristic; and an image forming unit that forms an image on the recording medium according to the output image data at the image forming speed.
In the image forming apparatus, the gradation conversion characteristic selector can set the optimal gradation conversion characteristic even if the speed setter sets the faster image forming speed to change the reproduced density range. As a result, the gradation can be correctly maintained in the reproduced image while the image forming speed is increased.
The above-mentioned object may be also achieved by an image forming apparatus that forms an image on a recording medium, the image forming apparatus that includes: a judging unit that judges a density characteristic of input image data; a speed setter that sets an image forming speed according to a judgement result of the judging unit; a data converter that converts gradation values of the input image data to generate output image data; and an image forming unit that forms an image on the recording medium according to the output image data at the image forming speed.
In the image forming apparatus, the density characteristic of the input image data is judged and the image forming speed is changed according to the judgement result. Accordingly, the image forming speed is increased when it is judged that the input image data has a density characteristic that has little effects on gradation reproduction even if the image forming speed is increased. By doing so, the gradation can be correctly maintained in the reproduced image while the image forming speed is increased.